The present invention is related to a knife as a hand tool, and more particularly to an automatically loadable and blade-stabilizing utility knife.
The conventional utility knifes as hand tools have been developed by several stages.
The early utility knife has a housing composed of two substantially symmetrical casings locked together by screws. A blade is accommodated in the housing. The housing has an opening at front end and is equipped with a driving unit for pushing the blade to protrude from the front end and pulling the blade back into the housing. When the blade is worn out, a user must unscrew the screws to open the housing for changing the direction of the blade or replacing the blade. It is troublesome and apt to get hurt to replace the blade.
An improved utility knife has a hooking unit disposed between the two casings for easily opening the housing. However, a user still needs to change the direction of the blade or replace the blade with hands. Therefore, the user is still easy to get hurt when replacing the blade.
A further improved utility knife has a switch disposed at the front end of the housing. By means of rotating or pressing the switch, the blade can be extracted out of the housing to change the direction of the blade or replace the blade. Then the blade is inserted into the front end of the housing. With such structure, it is  unnecessary to open the housing.
According to the above structures, only one blade is received in the housing. This is inconvenient in use. For example, in the case that no spare blade is available, it is impossible to further use the utility knife. In some utility knifes, a spare blade is placed in the housing. However, it is still necessary for a user to open the housing and replace the blade with hands.
Two kinds of advanced and automated utility knifes have been developed. Such utility knifes carry spare blades with themselves.
U.S. Pat. No. 5,604,984 discloses a first kind of utility knife carrying spare blade with itself. Such utility knife has a blade rest seat rotatable about a long axis of the housing. A series of blades are circumferentially arranged in the blade rest seat. The utility knife is designed with several switches for replacing the blades inside the housing. Such utility knife has some shortcomings as follows:                1. The structure of such utility knife is complicated. Also, it is troublesome and uneasy to operate such utility knife. In addition, the manufacturing cost for such utility knife is relatively high.        2. The spherical blade rest seat leads to enlarged handle of the utility knife. It is uneasy and uncomfortable to hold the handle.        3. Referring to FIGS. 1A and 1B, the blade used in such utility knife is not the one with standard commercial specification as shown in FIG. 1A. Instead, the blade used in such utility knife is a trapezoid one as shown in FIG. 1B.  The angle contained between the bottom side and the lateral side of the blade is smaller than the standard specification so that the stress strength of such blade is less. Especially when cutting a caplet, the blade is very apt to break. Therefore, such utility knife is not popular after released.        
FIGS. 2 to 4 show the second kind of utility knife carrying spare blade with itself. One side of the housing 91 is formed with a window 911. A cover 92 is pivotally latched in the window 911. Several overlapping blades 93 are transversely placed in the housing 91 through the window 911. In the case that the utility knife drops down, the cover 92 is apt to bound away and the blades 93 may slip out of the window 911. Furthermore, with such structure, a user can hardly pinch two plane faces of the blades 93 with fingers to load the blades 93. Instead, the user must pinch the sharp edges of the blades 93 to load the blades 93. Therefore, the fingers of the user are apt to be cut.
Moreover, after the blades 93 are placed into the housing 91, the blades 93 are uneasy to tidily overlap each other and snugly attach to a feeding unit 94 disposed in the housing. Therefore, the feeding unit 94 often fails to engage with the blade 93. This leads to malfunction of the utility knife.
On the other hand, the feeding unit 94 has an engaging block 941 for engaging with the blade. Two springs 942 are arranged on lateral sides of the engaging block 941 for pushing the engaging block 941 to engage in the notches 931 of the blade 93. The utility knife serving as a hand tool must be such designed that a user's hand can snugly hold the handle of the utility knife. Therefore, the housing 91 of the utility knife is fixed and thus the interior space of the housing 91 is limited. Accordingly, the two springs 942 are very small. It is uneasy to assemble so  small components of the utility knife. In addition, once the springs 942 are bent or inclined, the engaging block 941 cannot effectively engage with the blade 93. This will lead to malfunction of the utility knife. Furthermore, when feeding the blade 93 to protrude from the front end of the housing 91, the blade 93 is simply engaged with the engaging block 941 without any other design for stabilizing the blade 93. Therefore, the blade 93 can be hardly smoothly fed. In addition, the blade 93 is hung on the movable feeding unit 94. A gap must exist between the feeding unit 94 and the housing 91 for easily moving the feeding unit 94. As a result, the structure is unstable and when using the utility knife, the blade 93 is apt to swing.